Conventional memory components can be classified into two categories: Random Access Memory (RAM) and Read Only Memory (ROM). The data stored in RAM will disappear immediately after power supply is turned off, and thus RAM is also known as volatile memory. Contrarily, the data stored in ROM can be retained even without power supply. Therefore, ROM is also known as non-volatile memory. With the development of consumer electronics, ROM becomes more sophisticated. Various types of ROM emerge, such as Programmable ROM (PROM), Erasable & Programmable ROM (EPROM), Electrically Erasable &Programmable ROM (EEPROM), Flash Memory, and the likes. The flash memory becomes more and more popular because of noiseless, faster access, small in size, shock proof, and moisture proof relative to conventional products. Therefore, the flash memory is widely used in digital camera, mobile phone, MP3 player, and the likes.
The flash memory has a grid of bit lines and word lines with a cell that has two transistors at each intersection. The two transistors are separated from each other by a thin oxide layer. One of the transistors is known as a floating gate and the other one the control gate. The floating gate, located between the silicon substrate and the control gate, is surrounded by an insulation layer and does not link to word lines, bit lines, or any other wires. While data is written into the flash memory, a voltage higher than the drain voltage is applied to the control gate, thus hot electrons near the tunnel is produced. The hot electrons then accelerate and finally jump into the floating gate. The hot electrons then become trapped in the floating gate to complete data writing.
However, the portable electronic products become smaller and smaller while the requirement of the data storage capacity becomes larger and larger. Therefore, the main target is to simultaneously raise the product reliability and cell density of the memory. Several kinds of multi-bit non-volatile memory structures, such as the SONOS structure with nitride layers as the storage nodes, attempt to provide the above target. However, the cell density of SONOS structure is restricted by the disturbance effect of the storage electrons. Furthermore, the characteristics of the nitride layer lead to poor data retention. Thus, long-term stability of the memory is decreased when the nitride layer is used as the storage node.
On the other hand, the stacked-type non-volatile memory structure in accordance with the present invention provides long-term stability, high cell density, and a manufacture method with self-aligned floating gate to minimize the photolithography processes.